1. Field of the Invention
The present invention relates to a developer for developing electrostatic latent images. The developer is adapted for use in image recording apparatus such as copying machines and printers. More particularly, the invention relates to an electrostatic latent image developer useful in an image recording apparatus wherein the electrostatic latent image developer is controlled based on image information retrieved as electric signals to form images directly on a recording member.
2. Description of Related Art
Electrophotographic image recording methods and electrostatic image recording methods are widely used in copying machines, printers and the like. Most electrostatic latent image developers are used in these fields. Developers used in these methods are broadly classified into two groups. The first group of developers includes non-magnetic one-component developers used to develop only with colored particles comprising at least a resin ingredient and a pigment. The second group includes two-component developers which comprise a magnetic one-component developer comprising, at least, magnetic powder such as magnetite and a resin, and a mixture of particles called a carrier, such as magnetite, glass beads and the like, which are apparently greater in size than colored particles of the non-magnetic one-component developer, with colored particles as used in the non-magnetic one-component developer.
As an intermediate developer between the one-component developer and the two-component developer, a 1.5-component developer is known wherein a carrier is mixed with the magnetic one-component developer. The two-component developer comprises 90 to 95% of a carrier and 5 to 10% of a developer (hereinafter referred to as "a toner") including colored particles therein. The one-component developer, whether non-magnetic or magnetic, is completely free of any carrier and is constituted only of a toner. On the other hand, the 1.5-component developer comprises 30 to 90% of a carrier and 10 to 70% of a toner, so that this developer is called a 1.5-component developer in the sense that it is an intermediate between the one-component developer and the two-component developer.
In ordinary electrophotographic methods using a one- or two-component developer, an electrically latent image is formed on a photosensitive material made of a photoconductive material and the latent image is developed to provide a visible image. The visible image is then fixed, as it is or after transfer, on a recording medium on which the image is to be formed by the application of heat or pressure.
However, known electrophotographic and electrostatic recording methods have various problems when a high degree of image resolution is needed. To form images of high resolution, the particle size should be small. The toner produced by a grinding procedure limits the minimum particle size to approximately 7 .mu.m. Forming a finer sized toner is almost impossible so far as fine powder is removed, thus placing a limit on image resolution. On the other hand, when a toner is produced through polymerization, toner particles having a size not larger than 7 .mu.m may be formed, with the likelihood of a high degree of resolution. However, polymer particles deposited on the photosensitive material made of a photoconductive material are not easily removed in existing image recording apparatus, thereby causing a failure in cleaning.
With the charging system of a known two-component developer using a carrier, the amount of toner deposited is limited relative to the carrier's specific surface area. It has been accepted that the limit of toner concentration is about 5%. With two-component development systems using a low toner concentration relative to carrier, toner consumed for the development has to be invariably supplied to a development unit in an amount corresponding to the consumption in order to keep a constant toner concentration. To this end, if an image consuming a large amount of toner is outputted, the toner cannot be supplemented satisfactorily, thereby forming an image with a low density in a subsequent output cycle.
The 1.5-component developer is made of a mixture of a carrier and a magnetic toner wherein the concentration of the toner in the developer exceeds 5%. Since the toner is magnetic in nature and has a sufficient ability to function as a one-component magnetic toner, mixing with a carrier is not always necessary. More particularly, with the 1.5-component developer, the carrier is contained so that the one-component magnetic toner is made up for with respect to the chargeability and image quality is improved.
However, known toners have a bulk density of not larger than 0.4 g/cc or a cohesion of not larger than 20%. This means that the contact area is so small that the toner is not subjected to triboelectric charging to an extent sufficient to complement the charge of the toner.